A three-dimensional analysis of protein redistribution in the extracapillary space (ECS) of ultrafiltration membrane hollow-fiber bioreactors (HFBRs), used for mammalian cell culture, is presented. Homogeneous distribution of growth factor proteins in the ECS is essential for a successful startup and efficient operation of HFBRs. The ECS protein transport under most startup conditions of practical interest is strongly influenced by gravity and represents a complex interaction of forced- and free-convective phenomena. These effects were investigated using a comprehensive porous medium model (PMM) that accounts for local variations of fluid density, fluid viscosity, and osmotic pressure resulting from time-dependent changes in the protein concentration field. In addition, the model considers the influence of fiber free manifolds, which are adjacent to the fiber bundle and are accessible to ECS proteins and cells. The PMM predictions of the ECS protein distributions at different bioreactor orientations were in good agreement with the observed distributions of a colored test protein in an experimental HFBR cartridge. The results of this study can provide useful insights for optimizing HFBR operation strategies. (C) 2004 American Institute of Chemical Engineers.